THE NATIONAL GEOGRAPHIC MAGAZINE
Photograph from Mt. Wilson Observatory
TIE 20-FOOT INTERFEROMETER ATTACHED TO THE I00-INCH MT. WILSON TELESCOPE
This is the instrument with which the diameters of Betelgeuse (215,ooo,ooo miles),
Arcturus (21,ooo,ooo miles), and Antares (400,ooo,ooo miles) were measured. Two small
mirrors on the upper edge and near the ends of the beam gathered the two pencils of light
whose "interference" with one another was made to tell the story of the stars' diameters.
Thus he gets a triangle the length of
whose base he knows, and the angles the
two remaining sides form with the base
line. The rest is a high-school problem
in mathematics.
In the determination of the distance of
the moon, a base line from a given point
in America to another given point in
France was of sufficient length, and in
the measurement of distances of the closer
planets the diameter of the earth repre
sented the base line.
But when it comes to finding a base line
which will give any appreciable angle in
the measurement of star distances, no
terrestrial distance will suffice.
GETTING A LINE ON STELLAR NEIGHBORS
After patiently groping for such a base
line, the diameter of the earth's orbit was
taken. A star was viewed from one side
of the orbit and the angle for the one side
of the triangle measured. Six months
later it was viewed from the other side of
the orbit, and the angle for the other line
arrived at.
Imagine constructing a triangle with a
base one inch long and two sides eight
miles long, and then measuring the angles
at which the two sides depart from the
base!
That was the sort of problem Bessel
had when, in 1838, he measured the dis
tance of the first star so studied, although
his base line was 186,000,000 miles long
(the diameter of the orbit of the earth
around the sun). It took years for him
to make the calculations by which the dis
tance of 61 Cygni, in the constellation of
the Swan, was fixed at forty trillion miles.
So laborious did such work prove that
the distances of only 60 stars had been
fixed up to 1900.
After that the Yerkes Observatory un
dertook to fix the distances of other close
stars by photographic methods. Hold your
pencil in an upright position before your
eyes, about eight inches away. and then
look at a picture on the wall ahead of you.
Note the change of position of the pencil
with reference to the distant picture when
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